1. Field of the Invention
This invention relates to a method for the production of ethylene oxide. More particularly, it relates to a method for producing ethylene oxide by the catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas, which method is characterized by the use of hot water as a heat medium for the reaction apparatus.
2. Description of the Prior Art
In the process for catalytic vapor phase oxidation of a hydrocarbon, specifically the catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst for the production of ethylene oxide, the catalytic vapor phase reaction is carried out by mixing a hydrocarbon-containing gas containing saturated hydrocarbons such as methane and ethane, nitrogen, carbon dioxide, argon, and oxygen in addition to ethylene with a molecular oxygen-containing gas such as air, oxygen-enriched air, or pure oxygen in a prescribed ratio and introducing the resultant mixed gas into a reactor packed with a silver catalyst. Like virtually all hydrocarbons, ethylene reacted with molecular oxygen in the presence of a silver catalyst induces exothermic reactions represented by the following formulas: EQU C.sub.2 H.sub.4 +1/2O.sub.2 .fwdarw.C.sub.2 H.sub.4 O+29 kcal/mol(1) EQU C.sub.2 H.sub.4 +3O.sub.2 .fwdarw.2CO.sub.2 +2H.sub.2 O316 kcal/mol(2)
For the purpose of enhancing the yield of ethylene oxide, these exothermic reactions ought to be controlled so as to allow the reaction of Formula (1) to proceed in a large ratio. In this connection, studies are being pursued regarding silver catalyst and reaction method. By the existing technical standard, however, the simultaneous occurrence of the reaction of Formula (2), namely the complete oxidation of ethylene which entails evolution of a large amount of heat of reaction, cannot be avoided.
Besides Formulas (1) and (2) mentioned above, there exists another formula which affects the yield of ethylene oxide. It is the isomerization of the produced ethylene oxide into acetaldehyde which proceeds as indicated by Formula (3) shown below. ##STR1##
No matter how high the selectivity in the conversion of ethylene to ethylene oxide may be, the degradation of the yield of ethylene oxide inevitably ensues when the isomerization of the produced ethylene oxide into acetaldehyde notably occurs.
Acetaldehyde has a boiling point very close to that of ethylene oxide. Therefore, isolation of ethylene oxide from acetaldehyde by such a process as distillation, calls for a large amount of energy even when the acetaldehyde is contained in an extremely small amount. Further, since the number of trays in the distillation column is large and the reflux ratio therein is high and consequently the cost of equipment is high, an increase in the acetaldehyde content affects the cost of purification to a large extent.
The reactor outlet gas has a high temperature and, therefore, is made to exchange heat with the reactor inlet gas for recovery of heat. The cooled reaction product gas gives rise to a drain which mainly contains water, ethylene oxide, and acetaldehyde. Since this drain has a high acetaldehyde content, the isolation of ethylene oxide from the drain is extremely difficult to perform. The drain, therefore, leads to a decline in the yield of ethylene oxide.
The cooled reaction product gas is introduced into an absorption column and absorbed therein by an absorption liquid mainly containing water and ethylene glycol and the absorption liquid now containing the absorbed reaction product gas is forwarded to the next step for purification. In this case, ethylene oxide and water partly react in the absorption liquid to form ethylene glycols. For the purpose of maintaining the concentration in the system, the absorption liquid containing the ethylene glycols is partly removed from the system and subjected to purification and recovery of ethylene glycols. Due to the impurities originating in the aldehyde contained in the absorption liquid, the recovered ethylene glycols have degraded qualities, particularly in uv absorbance. Even for the purpose of preventing the decrease of qualities, the content of these aldehydes in the absorption liquid is preferably low.
Minimization of the isomerization of ethylene oxide into acetaldehyde, therefore, constitutes in itself one of the major tasks assigned to the production of ethylene oxide.
Since the demand for ethylene oxide has been growing in recent years and for the purpose of lowering the cost of production of ethylene oxide, plans for enlarging the existing plants for the production of ethylene oxide are gaining in impetus. Thus, the amount of the heat of reaction produced in the reactors per ethylene oxide production plant is large as plainly surmised from the aforementioned formulas of reaction, though the enhancement of yield has been achieved to a considerable extent. The question on how the large amount of the heat of reaction ought to be safely and effectively recovered for reuse poses a problem of safety and economy for the production of ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen.
Generally, in the catalytic vapor phase oxidation of a hydrocarbon with molecular oxygen, the method of passing the reaction product gas through a cooling zone filled with a packing for the purpose of curbing secondary reactions has been known to the art JP-B-39-17,254(1964). The practice of applying this method to the production of ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen has also been known to the art. GB-1,449,091 and GB-1,449,092, for example, teach a method which comprises causing the reaction product obtained in a reaction zone to be passed through a cooling zone which adjoins or does not adjoin the reaction zone and contains or does not contain an inert packing. U.S. Pat. No. 4,061,659 discloses a method which comprises passing the reaction product through a cooling zone packed with inert refractory particles having a surface area of not more than 1 m.sup.2 /g.
The reaction apparatuses which are used for these methods adopt a procedure which comprises circulating a high boiling heat medium such as Dowtherm (a proprietary product of the Dow Chemical Company) or a low boiling heat medium such as water to the reaction zone for removal of heat and using a high boiling heat medium such as Dowtherm in the cooling zone. These methods have the disadvantage in that the thermal media are high boiling dangerous substances which are difficult to handle and the apparatuses are complicated and not easy to operate.
Heretofore, as a reactor for the production of ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen, a shell-and-tube exchanger type reactor which is so constructed as to cool the reaction tubes of the reactor by forcibly circulating with a pump a high boiling heat medium such as Dowtherm between the exteriors of the reaction tubes and a heat exchanger adapted to form steam thereby removing the heat of reaction, cause the heat medium consequently heated to an elevated temperature to give the heat thereof to the water through the heat exchanger and make the water generate steam, and recirculating the consequently cooled heat medium and cooling the exteriors of the reaction tubes again has been known to the art.
Besides, a shell-and-tube exchanger type reactor which is so constructed as to effect formation of steam directly on the shell side of the reactor when a vaporizable heat medium such as water is used and, therefore, obviate the necessity for using a heat exchanger adapted to form steam as described above and permit spontaneous circulation when hot water is used and, therefore, obviate the necessity for installing a pump has been known to the art.
JP-B-1-56,070(1989) proposes a method for the catalytic vapor phase oxidation of ethylene, which uses a shell-and-tube exchanger type reactor having a reaction zone and a cooling zone adjoining each other and effects the oxidation by first supplying hot water to the cooling zone, cooling the reaction product gas cooling zone, causing the hot water to be further heated and discharged out of the reactor, flushing the hot water thereby inducing gas-liquid separation, then supplying the hot water to the reaction region thereby removing the heat of reaction by cooling, allowing the hot water to boil and depart from the reactor, flushing the hot water for gas-liquid separation, circulating the hot water to the reaction zone, recovering the steam, and accomplishing effective recovery of heat.
In these conventional reactors for the production of ethylene oxide, however, the effect of curbing the occurrence of aldehydes cannot be amply fulfilled because the hot water of a temperature in the range between 100.degree. C. and 150.degree. C. is first passed through the cooling zone provided for a vertical shell-and-tube exchanger type reactor and then used for cooling the reaction zone and, as a natural consequence, the amount of the hot water passed through the cooling zone is very small as compared with that passed through the reaction zone and the reactor is destined to be cooled unevenly. Moreover, the baffle plate portion of the cooling zone, the lower tube plate contiguous to the hot water in the cooling zone, and the lower tube plate contiguous to the reaction product gas have the possibility of generating thermal stress and causing vibration.
This invention has been produced to solve the problems encountered by the conventional reactors as described above.
An object of this invention is to provide a method for the production of ethylene oxide which eliminates the possibility of the baffle plate portion of the cooling zone of the vertical shell-and-tube exchanger type reactor for the production of ethylene oxide, the lower tube plate contiguous to the hot water of the cooling zone, and the lower tube plate contiguous to the reaction product gas generating thermal stress and inciting vibration, promotes enhancement of thermal efficiency, and heightens the effect of curbing the isomerization of ethylene oxide into aldehydes as impurities.